The World Wide Web, which is a collection of interconnected and cross-linked web pages located on computers, has become an important source of information. Users employ applications known as browsers to access these web pages. Since users frequently access the same web pages repeatedly, especially web pages that they have recently used, most browsers keep track of the web pages that the user has previously accessed using a technique known as a history. Previous browsers have used a linear history in chronological order that the user could navigate through using forward and back buttons, which are analogous to the rewind and fast-forward buttons on a VCR (video cassette recorder).
For example, a user might first visit the home page of a favorite baseball team and then follow a link on that home page to a second page with a particular player's batting average, and then follow another link on the second page to a third page containing an magazine article about the player. Subsequently, the user selects the back button on the browser twice to return to the home page of the baseball team instead of needing to type in the address of the home page.
A problem with this linear history is that users can visit a large number of web pages, which are confusing to view in a linear history, and the forward and back buttons are inefficient and cumbersome way to navigate through multiple web pages. Further, the problems of a linear history are not confined to pages accessed via a web browser, but also apply when a succession of data of other types is accessed over a period of time.